U.S. patent number 8,461,518 [Application Number 13/145,786] was granted by the patent office on 2013-06-11 for method of collecting calibration data in radiation tomography apparatus.
This patent grant is currently assigned to Shimadzu Corporation. The grantee listed for this patent is Masaharu Amano, Yoshihiro Inoue, Tetsuro Mizuta, Atsushi Ohtani, Kazumi Tanaka. Invention is credited to Masaharu Amano, Yoshihiro Inoue, Tetsuro Mizuta, Atsushi Ohtani, Kazumi Tanaka.
United States Patent |
8,461,518 |
Mizuta , et al. |
June 11, 2013 |
Method of collecting calibration data in radiation tomography
apparatus
Abstract
This invention has one object to provide a method of collecting
calibration data in radiation tomography apparatus that allows
reliable collection of calibration data with a wide detector ring.
In order to achieve this purpose, in the method of collecting
calibration data in radiation tomography apparatus according to
this invention, the number of coincidence events is obtained while
the phantom that emits annihilation gamma-ray pairs moves as to
pass through an inner hole of the detector ring. Such configuration
dose not need the phantom having a width equal or larger than the
detector ring, and may realize reliable collection of calibration
data. Moreover, it may be considered that annihilation gamma-ray
pairs have been emitted in uniform property without depending on
positions of the detector ring. As a result, calibration data that
is more suitable for removal of a image artifact may be
obtained.
Inventors: |
Mizuta; Tetsuro (Kyoto,
JP), Inoue; Yoshihiro (Kyoto, JP), Amano;
Masaharu (Ibaraki, JP), Tanaka; Kazumi (Otsu,
JP), Ohtani; Atsushi (Kyoto, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
Mizuta; Tetsuro
Inoue; Yoshihiro
Amano; Masaharu
Tanaka; Kazumi
Ohtani; Atsushi |
Kyoto
Kyoto
Ibaraki
Otsu
Kyoto |
N/A
N/A
N/A
N/A
N/A |
JP
JP
JP
JP
JP |
|
|
Assignee: |
Shimadzu Corporation (Kyoto,
JP)
|
Family
ID: |
42355608 |
Appl.
No.: |
13/145,786 |
Filed: |
January 23, 2009 |
PCT
Filed: |
January 23, 2009 |
PCT No.: |
PCT/JP2009/000251 |
371(c)(1),(2),(4) Date: |
July 21, 2011 |
PCT
Pub. No.: |
WO2010/084528 |
PCT
Pub. Date: |
July 29, 2010 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20110278443 A1 |
Nov 17, 2011 |
|
Current U.S.
Class: |
250/252.1 |
Current CPC
Class: |
G01T
1/1644 (20130101); G01T 1/1648 (20130101); G01T
1/2985 (20130101) |
Current International
Class: |
G01D
18/00 (20060101) |
Field of
Search: |
;250/252.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
2-262086 |
|
Oct 1990 |
|
JP |
|
10-2965 |
|
Jan 1998 |
|
JP |
|
2001-208849 |
|
Aug 2001 |
|
JP |
|
2003-536107 |
|
Dec 2003 |
|
JP |
|
Other References
International Search Report for the Application No.
PCT/JP2009/000251 mailed Mar. 17, 2009. cited by applicant.
|
Primary Examiner: Taningco; Marcus
Attorney, Agent or Firm: Cheng Law Group, PLLC
Claims
The invention claimed is:
1. A method of collecting calibration data in radiation tomography
apparatus having unit detecting rings with radiation detecting
elements arranged annularly for detecting radiation, a coincidence
device for counting a number of coincidence events as a number of
times that two different radiation detecting elements detect
radiation coincidentally, and a calibration data generation device
for generating calibration data based on the number of coincidence
events, and further having a detector ring with the unit detector
rings laminated such that central axes thereof conform to one
another, wherein the number of coincidence events is obtained while
a phantom emitting annihilation radiation pairs moves within an
inner hole of the detector ring; wherein a direction where the
detector ring extends is an extension direction, the phantom moves
relative to the detector ring along the extension direction, and
the phantom has a length in the extension direction that is shorter
than detector ring in the extension direction.
2. The method of collecting calibration data in radiation
tomography apparatus according to claim 1, wherein the phantom is
moved with a phantom moving device that moves the phantom and a
phantom control device that controls the phantom moving device.
3. The method of collecting calibration data in radiation
tomography apparatus according to claim 1, wherein, the phantom
reciprocates relative to the detector ring.
4. The method of collecting calibration data in radiation
tomography apparatus according to claim 1, wherein the phantom has
a speed that is the fastest at the time of starting movement, and
gradually slows as the phantom moves.
5. The method of collecting calibration data in radiation
tomography apparatus according to claim 1, wherein the coincidence
device counts the number of coincidence events only when a distance
between two radiation detecting elements in the extension direction
is equal or less than a given length, and the phantom has a length
in the extension direction that is longer than the given
length.
6. The method of collecting calibration data in radiation
tomography apparatus according to claim 2, wherein the phantom
reciprocates relative to the detector ring.
7. The method of collecting calibration data in radiation
tomography apparatus according to claim 2, wherein the phantom has
a speed that is the fastest at the time of starting movement, and
gradually slows as the phantom moves.
8. The method of collecting calibration data in radiation
tomography apparatus according to claim 3, wherein the phantom has
a speed that is the fastest at the time of starting movement, and
gradually slows as the phantom moves.
9. The method of collecting calibration data in radiation
tomography apparatus according to claim 2, wherein the coincidence
device counts the number of coincidence events only when a distance
between two radiation detecting elements in the extension direction
is equal or less than a given length, and the phantom has a length
in the extension direction that is longer than the given
length.
10. The method of collecting calibration data in radiation
tomography apparatus according to claim 3, wherein the coincidence
device counts the number of coincidence events only when a distance
between two radiation detecting, elements in the extension
direction is equal or less than a given length, and the phantom has
a length in the extension direction that is longer than the given
length.
11. The method of collecting calibration data in radiation
tomography apparatus according to claim 4, wherein the coincidence
device counts the number of coincidence events only when a distance
between two radiation detecting elements in the extension direction
is equal or less than a given length, and the phantom has a length
in the extension direction that is longer than the given length.
Description
TECHNICAL FIELD
This invention relates to a method of collecting calibration data
in radiation tomography apparatus that images radiation.
Particularly, this invention relates to a method of collecting
calibration data in radiation tomography apparatus having block
radiation detectors arranged in a ring shape.
BACKGROUND ART
In medical fields, radiation emission computed tomography (ECT:
Emission Computed Tomography) apparatus is used that detects an
annihilation radiation (for example, gamma rays) pair emitted from
radiopharmaceutical that is administered to a subject and is
localized to a site of interest for obtaining sectional images of
the site of interest in the subject showing radiopharmaceutical
distributions. Typical ECT equipment includes, for example, a PET
(Positron Emission Tomography) device and an SPECT (Single Photon
Emission Computed Tomography) device.
A PET device will be described by way of example. The PET device
has a detector ring with block radiation detectors arranged in a
ring shape. The detector ring is provided for surrounding a
subject, and allows detection of radiation that is transmitted
through the subject.
Such radiation detector arranged in the detector ring of the PET
device is often equipped that allows position discrimination in a
depth direction of a scintillator provided in the radiation
detector for enhanced resolution. First, description will be given
of a configuration of a conventional PET device. As shown in FIG.
10, a conventional PET device 50 includes a gantry 51 with an
introducing hole that introduces a subject, a detector ring 53
having block radiation detectors 52 for detecting radiation being
arranged inside the gantry 51 as to surround the introducing hole,
and a support member 54 provided as to surround the detector ring
53. Each of the radiation detectors 52 has a bleeder unit 55 with a
bleeder circuit in a position between the support member 54 and
thereof for connecting the support member 54 and the radiation
detector 52.
The PET device determines annihilation radiation pairs emitted from
radiopharmaceutical. Specifically, an annihilation radiation pair
emitted from inside of a subject M is a radiation pair having
traveling directions opposite by 180 degrees.
In order to use such PET device 50, variation in sensitivity is
firstly obtained that is used for image correction. Specifically, a
cylindrical phantom as a generating source of annihilation
radiation pairs is inserted into the gantry 51, and the detector
ring 53 detects annihilation radiation pairs (see, for example,
Patent Documents 1.) Here, although annihilation radiation pairs
are uniformly emitted from the entire phantom, the detector ring 53
does not necessarily output a result that annihilation radiation
pairs are uniformly emitted from the entire phantom. That is
because detection sensitivity of radiation varies in each radiation
detecting element that forms the detector ring 53. Accordingly,
some image artifact may fall in an image having imaged distribution
of annihilation radiation pairs inside the gantry 51, which
corresponds to variation in detection sensitivity of annihilation
radiation pairs that the detector ring 53 uniquely has.
The image artifact is superimposed also on a radiological image
with a subject image falling therein. Image processing that cancels
the image artifact is performed to the radiological image, whereby
variation in sensitivity falling in the radiological image is
cancelled. Accordingly, a clear radiological image with only the
subject image falling therein may be obtained. That is, the phantom
as a radiation source is inserted in advance to the detector ring
53 prior to performance of tomography of the subject for
determination of variation in detection sensitivity. [Patent
Literature 1] Japanese Patent Publication No. H10-2965
DISCLOSURE OF THE INVENTION
Summary of the Invention
The conventional construction, however, has the following problem.
Specifically, the problem is that the conventional construction has
difficulty in controlling the phantom to be inserted into the
detector ring 53 having a large width. Recently, radiation
tomography apparatus has been developed having the wide detector
ring 53 as to cover the entire of the subject. In order to cancel
variation in sensitivity that such radiation tomography apparatus
uniquely has, a long and huge phantom is needed sufficient to fill
a through hole of the detector ring 53.
Pre-arrangement is performed to the phantom prior to insertion into
the detector ring 53. Specifically, the phantom is a
solution-filled receptacle, and the inside thereof is firstly
filled with a solution. Subsequently, a radioactive material is
added to agitate the solution. The wider detector ring 53 needs an
increased amount of the solution for filling the phantom, which
leads to difficulty in agitating the solution. Moreover, it is also
complicated to insert the phantom subjected to the pre-arrangement
into the detector ring 53.
This invention has been made regarding the state of the art noted
above, and its object is to provide a method of collecting
calibration data in radiographic apparatus that allows collection
of the calibration data reliably with a wider detector ring 53.
Means for Solving the Problem
This invention is configured as stated below to achieve the above
object. A method of collecting calibration data in radiation
tomography apparatus according to this invention is provided having
unit detecting rings with radiation detecting elements arranged
annularly for detecting radiation, a coincidence device for
counting a number of coincidence events as a number of times that
two different radiation detecting elements detect radiation
coincidentally, and a calibration data generation device for
generating calibration data based on the number of coincidence
events, further having a detector ring with the unit detector rings
laminated such that central axes thereof conform to one another, in
which the number of coincidence events is obtained while a phantom
emitting annihilation radiation pairs moves within an inner hole of
the detector ring; wherein a direction where the detector ring
extends is an extension direction, the phantom moves relative to
the detector ring along the extension direction, and the phantom
has a length in the extension direction that is shorter than
detector ring, in the extension direction.
[Operation and Effect]
According to this invention, the number of coincidence events is
obtained while the phantom that emits annihilation radiation pairs
moves as to pass through the inner hole of the detector ring. Such
configuration dose not need the phantom having a width equal or
larger than that of the detector ring, and may realize reliable
collection of calibration data. Moreover, the phantom presents a
shape that emits annihilation radiation pairs uniformly to the
detector ring. However, actual annihilation radiation pairs
somewhat differ in irradiation property depending on positions of
the phantom. In the conventional configuration in which the phantom
does not move in the detector ring, annihilation radiation pairs to
be emitted differ in irradiation property depending on positions of
the phantom. Accordingly, variation in irradiation of the phantom
with annihilation radiation pairs will be superimposed on the
calibration data. On the other hand, according to this invention,
the phantom moves into the detector ring. Consequently, variation
in irradiation of annihilation radiation pairs in the phantom
becomes blurred in the calibration data. Moreover, it may be
considered that annihilation radiation pairs have been emitted in
uniform property without depending on positions of the detector
ring. As a result, calibration data more suitable for removal of
the image artifact may be obtained.
As in this configuration, the method of collecting calibration data
according to this invention is applicable to the case where the
detector ring extends over the phantom in the extension
direction.
Moreover, the phantom mentioned above is preferably moved with a
phantom moving device that moves the phantom and a phantom control
device that controls the phantom moving device.
[Operation and Effect]
The above configuration represents a specific aspect in movement of
the phantom. That is, this configuration includes each device for
moving the phantom. Accordingly, the phantom may be moved more
faithfully.
Moreover, the foregoing phantom preferably reciprocates relative to
the detector ring.
[Operation and Effect]
According to this configuration, the phantom may be placed in the
detector ring for a sufficiently long time, which results in
increase in number of coincidence events adopted for obtaining
calibration data. Moreover, the phantom moves complicatedly.
Accordingly, variation in radiation of the phantom with
annihilation radiation pairs becomes more blurred in the detector
ring. Consequently, calibration data may be obtained that is more
suitable for removal of the image artifact. The annihilation
radiation pair contained in the phantom is strongest when the
phantom is set into the detector ring, and is attenuated gradually.
According to this configuration, the phantom reciprocates in the
detector ring. Accordingly, it may be considered that annihilation
radiation pairs have been emitted in the detector ring in uniform
property.
Moreover, it is more preferable that the speed of the foregoing
phantom is the fastest at the time of starting movement, and
gradually slows as the phantom moves.
[Operation and Effect]
According to this configuration, it may be considered that
annihilation radiation pairs have been emitted in the detector ring
in uniform property. The annihilation radiation pair contained in
the phantom is strongest when the phantom is set into the detector
ring, and is attenuated gradually. Consequently, the dose of
annihilation radiation pairs per unit time to be emitted in the
detector ring differs between a movement starting position and a
movement termination position of the phantom. With this
configuration, control of the speed of the phantom may realize
decrease in dose of annihilation radiation pairs to be emitted in
the detector ring in the movement starting position of the phantom,
whereas may realize increase in dose of annihilation radiation
pairs to be emitted in the detector ring in the movement
termination position of the phantom.
Moreover, the foregoing coincidence device counts the number of
coincidence events only when a distance between two radiation
detecting elements in the extension direction is equal or less than
a given length. The phantom has a length in the extension direction
that is longer than the given length. Such configuration is
preferable.
[Operation and Effect]
With this configuration, data processing on the number of
coincidence events may significantly be reduced. Specifically, the
coincidence device counts the number of coincidence events only
when a distance between two radiation detecting elements in the
extension direction is equal or less than a given length. Where two
radiation detecting elements are far apart from each other in the
extension direction, the number of coincidence events is not
counted between such radiation detecting elements. That is because
a burden of the coincidence device may increase. Moreover, the
phantom has a length in the extension direction that is longer than
the given length. In other words, annihilation radiation pairs may
be emitted reliably between the two radiation detecting elements
that perform coincidence.
Effect of the Invention
In this invention, the number of coincidence events is obtained
while the phantom that emits annihilation radiation pairs moves as
to pass through the inner hole of the detector ring. Such
configuration dose not need the phantom having a width equal or
larger than the detector ring, and may realize reliable collection
of calibration data. Moreover, it may be considered that
annihilation radiation pairs have been emitted in uniform property
without depending on positions of the detector ring. As a result,
calibration data may be obtained that is more suitable for removal
of the image artifact.
Moreover, with this configuration, data processing on the number of
coincidence events may significantly be reduced. Specifically, the
coincidence device counts the number of coincidence events only
when a distance between two radiation detecting elements in the
extension direction is equal or less than a given length. Moreover,
in order to emit annihilation radiation pairs reliably between the
two radiation detecting elements that perform coincidence, the
phantom has a length in the extension direction that is longer than
the given length.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a functional block diagram showing a configuration of the
radiation tomography apparatus according to Embodiment 1.
FIG. 2 is a perspective view showing a configuration of a radiation
detector according to Embodiment 1.
FIG. 3 is a view showing a configuration of a detector ring
according to Embodiment 1.
FIGS. 4 and 5 are sectional views each showing a method of
obtaining an original map according to Embodiment 1.
FIGS. 6 to 8 are sectional views each showing a method of obtaining
a sectional image according to Embodiment 1.
FIG. 9 is a plan view showing a configuration according to one
modification of this invention.
FIG. 10 is a sectional cut-away view showing a configuration of the
conventional radiation tomography apparatus.
DESCRIPTION OF REFERENCES
9 . . . radiation tomography apparatus 12 . . . detector ring 12b .
. . unit detector ring 15 . . . bed moving mechanism (phantom
moving device) 16 . . . bed movement controller (phantom movement
control device) 21 . . . coincidence section (coincidence device)
26 . . . calibration data generation section (calibration data
generation device) 41 . . . phantom
BEST MODE FOR CARRYING OUT THE INVENTION
Description will be given hereinafter of the best mode of a method
of collecting calibration data in radiation tomography apparatus
according to this invention with reference to the drawings. Gamma
rays to be described hereinafter are an example of radiation in
this invention.
Embodiment 1
<Whole Configuration of Radiation Tomography Apparatus>
Each embodiment of radiation tomography apparatus according to this
invention will be described hereinafter with reference to the
drawings. FIG. 1 is a functional block diagram showing a
configuration of the radiation tomography apparatus according to
Embodiment 1. As shown in FIG. 1, the radiation tomography
apparatus according to Embodiment 1 includes a bed 10 for placing a
subject M on the back thereof, and a gantry 11 with a through hole
for surrounding the subject M. The bed 10 is provided as to pass
through an opening of the gantry 11. The bed 10 freely moves in and
out along a direction where the opening of the gantry 11 extends. A
bed moving mechanism 15 slides the bed 10 as above. Abed movement
controller 16 controls the bed moving mechanism 15.
The gantry 11 includes a detector ring 12 inside thereof that
detects annihilation gamma-ray pairs from the subject M. The
detector ring 12 is tubular and extends in a body axis direction z
of the subject M (corresponding to the extension direction of this
invention.) The detector ring 12 has a length of 1 m to 1.8 m. That
is, the detector ring 12 extends as to completely cover at least a
body portion of the subject M. A clock 19 sends out time
information to the detector ring 12. Time information about when
gamma rays were obtained is given to detection data outputted from
the detector ring 12, and inputted into a filter 20 mentioned
later.
The detector ring 12 has block radiation detectors 1 arranged in a
ring shape. It is assumed that a width per one radiation detector 1
is approximately 5 cm. Approximately twenty to thirty-six radiation
detectors 1 are to be arranged in the detector ring in the
z-direction. Next, simple description will be given of a
construction of the radiation detector 1. FIG. 2 is a perspective
view showing a configuration of the radiation detector according to
Embodiment 1. As shown in FIG. 2, the radiation detector 1 includes
a scintillator 2 that converts radiation into fluorescence, and a
light detector 3 that detects fluorescence. A light guide 4 is
provided between the scintillator 2 and the light detector 3 for
receiving fluorescence.
The scintillator 2 has two or more scintillation counter crystals
arranged in a two-dimensional array. Each of the scintillation
counter crystals is composed of Lu.sub.2(1-X)Y.sub.2XSiO.sub.5
(hereinafter referred to as LYSO.) The light detector 3 allows
determination about which scintillation counter crystal emits
fluorescence as well as of intensity of fluorescence and time when
fluorescence is generated. Here, the scintillator 2 having the
configuration of Embodiment 1 is only exemplification of an aspect
that may be adopted. Consequently, the configuration of this
invention is not limited to this.
Description will be given of a configuration of the detector ring
12. FIG. 3 is a view showing a configuration of the detector ring
according to Embodiment 1. The radiation detectors 1 are arranged
along an imaginary circle (exactly equilateral n-sided polygon) in
the detector ring 12. Accordingly, the scintillation counter
crystals are also arranged along an imaginary circle (exactly
equilateral n-sided polygon) to form a unit detector ring 12b as
shown in FIG. 3(a). The unit detector rings 12b are located in the
same position with respect to the z-direction. The unit detector
ring 12b is formed of scintillation counter crystals C (radiation
detecting elements) arranged along a circular ring. That is, the
unit detector ring 12b has the scintillation counter crystals
arranged in one row, which is based on an independent concept from
the radiation detector 1 arranged along the imaginary circle. Then,
as shown in FIG. 3(b), the unit detector rings 12b are connected to
one another in the z-direction to form the detector ring 12. The
unit detector ring 12b has a through hole at a center thereof. It
may be considered that the unit detector rings 12b are arranged
such that the through holes thereof are connected to one another to
form the detector ring 12.
According to Embodiment 1, the detector ring 12 is formed in a
circular ring shape by arranging around 100 radiation detectors 1.
Consequently, the through hole 12a is of 100-sided polygon, for
instance, seen thereof from the z-direction. In this case, two or
more unit detector rings 12b are connected as to share each central
axis thereof. The through hole 12a has a shape of a 100-sided
prism.
The radiation tomography apparatus 9 according to Embodiment 1
further includes each section for obtaining sectional images of the
subject M, as shown in FIG. 1. Specifically, the radiation
tomography apparatus 9 includes a filter 20 for extracting
effective data from detection data detected in the detector ring
12; a coincidence section 21 that receives the data determined as
the effective data in the filter 20 and performs coincidence of an
annihilation gamma-ray pair; an LOR specifying section 22 for
specifying an incident position of gamma rays in the detector ring
12 based on two pieces of gamma-ray detection data determined to be
an annihilation gamma-ray pair in the coincidence section 21; a
data storage section 23 for storing the detection data; a mapping
section 24 for generating a sectional image of the subject M; and a
calibration section 25 for performing calibration to the sectional
image of the subject M. The calibration section 25 removes a image
artifact falling in the sectional image with reference to an
original map stored in a calibration data storage section 34. Here,
the reason for provision of a counting region setting section 33
will be mentioned later. In addition, an MRD storage section 37
stores MRD, mentioned later. An input unit 38 inputs operator's
operations. For instance, the input unit 38 receives change of the
MRD, for instance.
The coincidence section corresponds to the coincidence device in
this invention. The calibration data generation section corresponds
to the calibration data generation device in this invention.
Moreover, the bed moving mechanism corresponds to the phantom
moving device in this invention. The bed movement controller
corresponds to the phantom movement control device.
The radiation tomography apparatus 9 according to Embodiment 1
further includes a main controller 35 for controlling each section
en bloc, and a display unit 36 for displaying a radiological image.
The main controller 35 is formed of a CPU, and performs execution
of various programs to realize the filter 20, the coincidence
section 21, the LOR specifying section 22, the data storage section
23, the mapping section 24, and the calibration section 25. The
above sections may each be divided into a controller that performs
their functions.
<Obtainment of Original Map>
Next, description will be given of a method of obtaining an
original map stored in the calibration data storage section 34. The
original map is mapping data containing mapped variation in
detection sensitivity of the annihilation radiation pairs that the
detector ring 12 uniquely has. In order to generate the original
map, firstly a phantom 41 is prepared that emits radiation. The
phantom 41 has an enough width to be placed on the bed 10, and is
cylindrical that extends in the z-direction. The phantom 41 has a
length in the z-direction that is characteristic of this
embodiment, which is to be mentioned later.
The phantom 41 is cylindrical that conforms to the shape of the
opening of the gantry 11, and has a hollow portion filled with a
solution. The phantom 41 has a movement direction relative to the
detector ring 12 that is the same direction as the extension
direction thereof. Firstly, the hollow portion of the phantom 41 is
filled with the solution, and then a radioactive material is added
thereinto. As for types of radioactive materials, a nuclide that
emits positrons to generate annihilation gamma-ray pairs is
selected. More particularly, the nuclide is preferably the same as
that used for radiopharmaceutical injected into the subject M. The
radiopharmaceutical is added into the phantom 41 filled with the
solution, and the hollow portion of the phantom 41 is enclosed.
Thereafter, the phantom 41 is shaken. In so doing, annihilation
gamma-ray pairs are to be emitted uniformly from the entire of the
phantom 41.
Subsequently, the phantom 41 is attached to the bed 10. Here, gamma
rays are attenuated due to the bed 10 when the phantom 41 is placed
on the bed 10. Consequently, a collection system is preferable in
which gamma rays emitted from the phantom directly reach the
detector using a phantom attachment jig, etc. Moreover, the phantom
41 has a length in the z-direction that is shorter than the
detector ring 12. The bed moving mechanism 15 slides the bed 10
having the phantom 41 carried thereon. Accordingly, as shown in
FIG. 4(a), a front end 41a of the phantom 41 passes the unit
detection ring 12c at one end of the detector ring 12. This point
in time is an initial position of the phantom 41.
The radiation tomography apparatus 9 collects calibration data
under a state where the phantom 41 is in the initial position
mentioned above. The detection data obtained in the detector ring
12 (data on fluorescence emitted from the scintillation counter
crystals) is sent to the filter 20. The filter 20 passes only
detection data in a counting region R obtained in the scintillation
counter crystals into the coincidence section 21, and disposes of
the other detection data. In so doing, needless calculation may be
omitted in the coincidence section 21, which leads to significant
simplification of the complicated coincidence calculation. The
counting region R in the initial position contains only
scintillation counter crystals belonging to the unit detection ring
12c. Specifically, it is shown as a slash region in FIG. 4(a).
The counting region setting section 33 sets the counting region R.
The filter 20 successively reads out a newest counting region R
from the counting region setting section 33.
Where detection data derived from two different scintillation
counter crystals is putted in a time window having a given temporal
width in the coincidence section 21, the data is assumed to be from
the annihilation gamma-ray pairs. The frequency of this is counted,
which corresponds to the number of coincidence events. Here,
temporal information that the clock 19 gives to the detection data
is used for determination of coincident property.
The LOR specifying section 22 specifies a direction where the
annihilation gamma-ray pairs are emitted. That is, the detection
data considered coincident by the coincidence section 21 also
contains positional information which scintillation counter crystal
emits fluorescence. The LOR specifying section 22 specifies an LOR
(Line of Response) as a line connecting the two scintillation
counter crystals, and sends out the LOR and the number of
coincidence events corresponding thereto to the data storage
section 23.
The bed 10 slides while radiation is detected as above.
Accordingly, a relative position of the phantom 41 and the detector
ring 12 is changed from the initial position, and the counting
region R is changed. Specifically, when the phantom 41 moves by a
width of the unit detector ring 12b in the z-direction from the
initial position, a region of the unit detector ring 12b adjacent
to the unit detector ring 12c is added to the counting region R. In
other words, data indicating a moving state of the bed 10 is sent
from the bed movement controller 16 to the counting region setting
section 33. The counting region setting section 33 expands the
counting region R in accordance with the data.
Subsequently, the counting region setting section 33 newly adds a
single unit detector ring 12b every time the phantom 41 moves by a
width of the unit detection ring 12b for expanding the counting
region R in the z-direction until the phantom 41 is in a state of
FIG. 4(b) where the detector ring 12 entirely covers with the
phantom 41. In other words, the scintillation counter crystal
facing the phantom 41 lies in the counting region R. In FIG. 4(b),
the counting region R has a width of eight scintillation counter
crystals. The number eight is obtained by adding the number one to
the MRD, mentioned later.
The phantom 41 moves by a width of the unit detector ring 12b from
the state of FIG. 4(b), and the counting region setting section 33
does not expand the counting region R any more, but shifts the
counting region R as to follow the phantom 41. That is, the
counting region setting section 33 removes the unit detector ring
12c from the counting region R, and resets the counting region R as
to newly add a single detector ring 12b located in a front end 41a
side of the phantom 41. The counting region setting section 33
shifts the counting region R every time the phantom 41 moves by a
width of the unit detector ring 12b. FIG. 5(a) shows the phantom 41
located in the middle of the detector ring 12. The counting region
R always lies in a section between the front end 41a and the rear
end 41b of the phantom 41 in the z-direction.
Subsequently, as the phantom 41 moves, the unit detection ring 12b
to be located behind the rear end 41b is to be removed from the
counting region R one after another. Finally, as illustrated in
FIG. 5(b), the rear end 41b of phantom 41 is located in unit
detection ring 12d which lies at one end of detector ring 12. This
state corresponds to the termination movement position of the
phantom 41.
In so doing, the scintillation counter crystals of the detector
ring 12 always lie in the counting region R during movement of the
phantom 41 from the initial position to the termination position.
Accordingly, coincidence is performed to every scintillation
counter crystal.
The data storage section 23 stores the LOR and the number of
coincidence events corresponding thereto. The mapping section 24
constructs the data to obtain an original map in which an interior
of the detector ring 12 is visible. The phantom 41 emits
annihilation radiation pairs uniformly. Accordingly, it may be
simply estimated that no image artifact appears in the original
map. In actual fact, however, it is not always the case. Some image
artifact appears in the original map under the influence of
variation in sensitivity among each scintillation counter
crystal.
The followings, other than the above variation in sensitivity, may
cause the image artifact. For instance, in the radiation detector
1, the scintillation counter crystal located on the side of the
scintillator 2 has a property that is difficult in detecting
radiation rather than that located in the middle of the
scintillator, which causes the image artifact. The factor causing
such image artifact is referred to as a transaxial block profile
factor. On the other hand, the longer and the narrower the LOR is,
the less detection sensitivity of gamma rays becomes, which
property leads to another image artifact. The factor leading to
such image artifact is referred to as a radial geometric
factor.
Moreover, the factors other than the foregoing factors include a
crystal interference factor. Where gamma-rays obliquely enter into
the scintillation counter crystals, or gamma-rays enter into the
DOI detector with information in the depth direction, gamma-rays
are not occasionally detected but pass through the scintillation
counter crystals adjacent to the phantom 41, and are detected in
crystals close to the crystals, i.e., apart from the phantom 41. In
this case, the scintillation counter crystal apart from the phantom
41 has detection sensitivity lower than that close to the phantom
41, which causes the image artifact. This is a crystal interference
factor. The original map also has the image artifact appearing
therein that is derived from these factors.
Calibration data is generated in the calibration data generation
section 26 based on the original map, and is stored in the
calibration data storage section 34. When the calibration data is
applied to the original map, the image artifact of the original map
is canceled.
<Operation of Radiation Tomography Apparatus>
Next, description will be given of operations of radiation
tomography apparatus 9 according to Embodiment 1. Upon conducting
of examinations with the radiation tomography apparatus 9 according
to Embodiment 1, firstly the subject M with radiopharmaceutical
administered thereto by injection in advance is laid on the bed 10.
Then, the bed 10 slides to move the subject M into the opening of
the gantry 11. From here, the annihilation gamma-ray pair emitted
from the subject M is detected.
Description will be given of a maximum ring difference (MRD) as an
important concept in detecting the annihilation radiation pair. The
LOR originally means a line connecting two different scintillation
counter crystals. As shown in FIG. 6, giving attention to a
scintillation counter crystal C8, numerous LORs may be considered.
However, all the LORs are not needed for obtaining the sectional
image. For instance, the LOR 100g connects the scintillation
counter crystal C8 and the scintillation counter crystal Dr. Both
scintillation counter crystals are far apart from each other in the
z-direction, and thus gamma-rays obliquely enter into the
scintillation counter crystals. Such gamma rays are difficult to be
detected. Performance of coincidence in the LOR 100g does not
contribute to clarifying the sectional image. Simultaneously,
calculation of coincidence should be simple. In terms of
suppression in number of LORs, an idea may be drawn that there is
no need for counting the LOR 100g from the beginning.
That is, when considered the LOR of the scintillation counter
crystal C8, only the scintillation counter crystals adjacent to the
scintillation counter crystal C8 in the z-direction are needed for
consideration. For instance, considered is the LOR of the
scintillation counter crystal C8 having a distance in the
z-direction of not more than seven scintillation counter crystals.
The number seven is a number of scintillation counter crystals for
controlling the number of LORs to be in consideration in performing
coincidence. This is referred to as MRD (Maximum ring difference.)
Specifically, it is assumed that MRD is seven. The LORs to be in
consideration are LOR 1g to LOR 15g. More generally, the LORs for
scintillation counter crystal C8 are only the LORs connecting any
of scintillation counter crystals in a region having a width of
fifteen scintillation counter crystals (i.e., of MRD.times.2+one
MRD) and the scintillation counter crystal C8.
Such selection of LORs is performed with the filter 20. Where two
scintillation counter crystals coincidentally detect gamma-rays,
the filter 20 passes detection data of the two scintillation
counter crystals into the coincidence section 21 when the two
scintillation counter crystals have a clearance therebetween in the
z-direction of the MRD or less, and disposes of the detection data
when the clearance is larger than the MRD.
The filter 20 sends detection data to the coincidence section 21,
the LOR specifying section 22, the data storage section 23, and the
mapping section 24. Operations of these are the same as the above,
and thus description thereof will be omitted. The sectional image
of the subject M constructed in the mapping section 24 is outputted
into the calibration section 25. In the calibration section 25,
data processing is performed for removing the image artifact
superimposed on the sectional image of the subject M based on the
calibration data stored in the calibration data storage section 34.
The display unit 36 displays a completion image obtained in this
way. As noted above, an inspection with the radiation tomography
apparatus 9 according to Embodiment 1 is to be completed.
<Regarding Length of Phantom>
Next, description will be given of a length of the phantom 41
distinctive in Embodiment 1. As described with FIG. 5(a), the
length of the phantom 41 is closely associated with the maximum
length of the counting region R. Accordingly, in Embodiment 1,
determination of the suitable maximum length of the counting region
R naturally leads to determination of the length of the phantom
41.
The filter 20 selects the LOR to be used for coincidence counting
upon inspection of the subject M. Moreover, the LOR is selected
through specifying the counting region R upon obtaining the
calibration data. Thus, the calibration data may be obtained most
effectively by corresponding selection of the LOR upon inspection
of the subject M to that upon obtaining the calibration data.
Description will be given of setting the length of the phantom 41
regarding the state of the art noted above. Firstly, the unit
detector ring 12c at one end of the detector ring 12 is to be
considered. FIG. 7 is a schematic view showing the LOR in the unit
detector ring at one end of the detector ring according to
Embodiment 1. Here, let the unit detector ring apart by seven unit
detector rings from the unit detector ring 12c be a detector ring
12e. As is apparent from FIG. 7(a), the LOR among the LORs of
scintillation counter crystal C8 in the unit detector ring 12c that
is used for inspection has eight types of LORs, i.e., seven types
of LORs 1g to 7g each connecting the scintillation counter crystals
C8 to the scintillation counter crystal apart by one to seven unit
detector rings, respectively, from the unit detector ring 12c, and
one type of LOR 8g connecting the scintillation counter crystal C8
to the scintillation counter crystal D8 in the unit detector ring
12c.
Here, a region K1 in FIG. 7(a) is a region occupied by the phantom
41 at one time when calibration data is obtained. The phantom 41
has a length equal or more than eight times the width of the unit
detector ring. Accordingly, every LOR 1g to LOR 8g will pass the
region K1. Assuming that the phantom 41 lies in the region K1, the
annihilation gamma-ray pairs are reliably emitted from the phantom
41 along the LOR 1g to LOR 8g. The length of eight times the width
of the unit detector ring (a width of MRD+one unit detector ring
12b) corresponds to the given length of this invention.
Secondary, the unit detector ring 12d at the other end of the
detector ring 12 is to be considered. Here, let the unit detector
ring apart by seven unit detector rings from the unit detector ring
12d be a detector ring 12f. As is apparent from FIG. 7(b), the LOR
among the LORs of scintillation counter crystal C8 in the unit
detector ring 12d that is used for inspection has eight types of
LORs, i.e., seven LORs 9g to 15g each connecting the scintillation
counter crystal C8 to the scintillation counter crystal apart by
one to seven unit detector rings, respectively, from the unit
detector ring 12d, and one LOR 8g connecting the scintillation
counter crystal C8 to the scintillation counter crystal D8 in the
unit detector ring 12d.
Here, a region K2 in FIG. 7(b) is a region occupied by the phantom
41 at one time when calibration data is obtained. The phantom 41
has a length equal or more than eight times the width of the unit
detector ring. Accordingly, every LOR 8g to LOR 15g will pass the
region K2. Assuming that the phantom 41 lies in the region K2, the
annihilation-gamma-ray pairs are reliably emitted from the phantom
41 along the LOR 8g to LOR 15g.
Next, description will be given of the scintillation counter
crystals located in the middle of the detector ring 12. This case
is seen from a conceptual view with a combination of FIG. 7(a) and
FIG. 7(b). FIG. 8 is a conceptual view showing the LOR in the unit
detector ring located in the middle of the detector ring according
to Embodiment 1. As is seen from FIG. 8(a), the LOR among the LORs
of the scintillation counter crystal C8 used for inspection has
fifteen types of LORs, i.e., (1) seven types of LORs 1g to 7g each
connecting the scintillation counter crystal C8 to the to the unit
detector ring located on the left side thereof (at the other end of
the detector ring 12) apart by one to seven unit detector rings,
respectively, from the scintillation counter crystal C8, (2) one
type of LOR 8g connecting the scintillation counter crystal C8 to
the unit detector ring belonging thereto, and (3) seven types of
LORs 9g to 15g each connecting the scintillation counter crystal C8
to the unit detector ring located on the right side thereof (at one
end of the detector ring 12) and apart by one to seven unit
detector rings, respectively, from the scintillation counter
crystal C8. A region K1 in FIG. 8(a) is a region occupied by the
phantom 41 at one time when calibration data is obtained. That is,
as for eight types of LOR 1g to LOR 8g described in the above (1)
and (2), the annihilation gamma-ray pairs are reliably emitted from
the phantom 41 along these LORs.
Moreover, a region K2 in FIG. 8(b) is a region occupied by the
phantom 41 at one time when calibration data is obtained. That is,
as for seven types of LOR 9g to LOR 15g described in the above (3),
the annihilation gamma-ray pairs are reliably emitted from the
phantom 41 along these LORs.
Where the scintillation counter crystal C8 is located adjacent to
the detector rings 12c and 12d in FIG. 7, some of the LORs 1g to
15g are not able to be drawn. This case may be described using FIG.
8. That is, where the scintillation counter crystal C8 is located
adjacent to the right end of the detector ring 12, it is only
necessary to delete some of the detector rings located on the right
side of the scintillation counter crystal C8 in FIG. 8(a). Where
the scintillation counter crystal C8 is located adjacent to the
left end of the detector ring 12, it is only necessary to delete
some of the detector rings located on the left side of the
scintillation counter crystal C8 in FIG. 8(b). As above, even when
the scintillation counter crystal C8 is located adjacent to one end
of the detector ring 12, the annihilation gamma-ray pairs are
always emitted reliably from the phantom 41 along the LORs for use
in inspection.
As above, in any cases, calibration data for every LOR used in
inspection is obtained, which leads to reliable removal of the
image artifact superimposed on the detection data obtained during
the inspection.
As noted above, Embodiment 1 has a configuration in which the
number of coincidence events are obtained while the phantom 41 that
emits annihilation radiation pairs moves as to pass through the
inner hole of the detector ring 12. Such configuration dose not
need the phantom 41 having a width equal or larger than that of the
detector ring 12, and may realize reliable collection of
calibration data. Moreover, the phantom 41 presents a shape that
emits annihilation gamma-ray pairs uniformly to the detector ring
12. However, actual annihilation gamma-ray pairs may somewhat
differ in irradiation property when the radiation source inside the
phantom 41 has poor uniformity. In the conventional configuration
in which the phantom 41 does not move in the detector ring 12,
annihilation radiation pairs to be emitted differ in irradiation
property depending on positions of the detector ring 12.
Accordingly, variation in irradiation of annihilation radiation
pairs in the phantom 41 will be superimposed on the calibration
data. On the other hand, according to Embodiment 1, the phantom 41
moves into the detector ring 12. Consequently, variation in
irradiation of the phantom 41 with annihilation radiation pairs
becomes blurred in the calibration data. Moreover, it may be
considered that annihilation radiation pairs have been emitted in
uniform property without depending on positions of the detector
ring 12. As a result, calibration data more suitable for removal of
the image artifact may be obtained. Here, in any cases where the
inner hole of the detector ring 12 is shorter or longer than the
phantom 41, movement of the phantom 41 relative to the detector
ring 12 may always remove variation in radiation of the phantom 41
with annihilation radiation pairs.
Moreover, with this configuration, data processing on the number of
coincidence events may significantly be reduced. Specifically, the
coincidence section 21 counts the number of coincidence events only
when a distance between two scintillation counter crystals in the
z-direction is equal or less than a given length (the maximum
length of the counting region R.) Where two scintillation counter
crystals are far apart from each other in the z-direction, the
number of coincidence events is not counted between such
scintillation counter crystals. That is because a burden of the
coincidence section 21 increases. Moreover, the phantom 41 has a
length in the z-direction that is longer than the given length. In
other words, the annihilation gamma-ray pair may be emitted
reliably between the two scintillation counter crystals that
perform coincidence.
This invention is not limited to the foregoing configurations, but
may be modified as follows.
(1) In each of the foregoing embodiments, the radiation detector 1
has a single scintillation counter crystal layer. This invention is
not limited to this embodiment. That is, as shown in FIG. 9(a), the
radiation detector 1 of multiple crystal layers may be adopted.
Such configuration may enhance detection sensitivity and a
discriminative capability of the incident position of gamma rays in
the radiation detector 1. According to the embodiment of FIG. 9(a),
four crystal layers having scintillation counter crystals arranged
two-dimensionally are laminated on a light guide 4. The number of
layers is not limited to this. A great majority of description in
Embodiment 1 may be used for operations of obtaining the original
map and generating the sectional image of the subject M in this
modification. A specific variation is that the LOR increases rather
than that in Embodiment 1. For instance, regarding the LOR of the
scintillation counter crystal C8 in FIG. 9(b), the LOR corresponds
to the LOR 15g in FIG. 8(a) is divided into four types of LORs,
i.e., (1) the LOR 15h connecting the scintillation counter crystal
C8 and an uppermost crystal layer, (2) the LOR 15i connecting the
scintillation counter crystal C8 and a second crystal layer, (3)
the LOR 15j connecting the scintillation counter crystal C8 and a
third crystal layer, and (4) the LOR 15k connecting the
scintillation counter crystal C8 and a fourth crystal layer. The
filter 20 passes detection data to each subsequent section without
discriminating the four layers. The LOR specifying section 22
performs data processing through discriminating the four
layers.
(2) In the foregoing embodiment, the phantom 41 only moves in one
direction. This invention is not limited to this configuration.
That is, the phantom 41 may once move from the state of FIG. 4(a)
to that of FIG. 5(b), and thereafter move back to the state of FIG.
4(a). In other words, the original map may be generated while the
phantom 41 reciprocates inside the detector ring 12. The effect
concerning this modification is as follows. The phantom 41 is
shaken prior to use to irradiate the detector ring 12 with the
annihilation gamma-ray pairs uniformly. However, actual
annihilation gamma-ray pairs differ in irradiation property
depending on positions of the phantom 41. In the conventional
configuration in which the phantom 41 does not move into the
detector ring 12, annihilation gamma-ray pairs to be emitted differ
in irradiation property depending on positions of the detector ring
12. Accordingly, variation in irradiation of annihilation gamma-ray
pairs in the phantom 41 will be superimposed on the calibration
data. According to this modification, the phantom 41 moves into the
detector ring 12. Consequently, variation in irradiation of
annihilation gamma-ray pairs in the phantom 41 becomes blurred in
the calibration data. Moreover, it may be considered that
annihilation gamma-ray pairs have been emitted in uniform property
without depending on positions of the detector ring. As a result,
calibration data more suitable for removal of the image artifact
may be obtained.
(3) Moreover, in the foregoing embodiment, reference has not been
made in particular to the speed of the phantom 41. In obtaining the
original map, the speed of the foregoing phantom 41 may be the
fastest at the time the phantom 41 starts moving, and that the
speed gradually slows as the phantom 41 moves. According to this
configuration, it may be considered that annihilation gamma-ray
pairs have been emitted in the detector ring 12 in uniform
property. The annihilation gamma-ray pair contained in the phantom
41 is strongest when the phantom 41 is set into the detector ring
12, and is attenuated gradually due to physical halftime.
Consequently, the dose of annihilation gamma-ray pairs per unit
time to be emitted in the detector ring 12 differs between the
movement starting position and the movement termination position of
the phantom 41. With this configuration, control of the speed of
the phantom 41 may realize decrease in dose of annihilation
gamma-ray pairs to be emitted in the detector ring 12 in the
movement starting position of the phantom 41, whereas the control
may realize increase in dose of annihilation gamma-ray pairs to be
emitted in the detector ring 12 in the movement termination
position of the phantom 41.
(4) In each of the foregoing embodiments, the scintillation counter
crystal is composed of LYSO. Alternatively, the scintillation
counter crystal may be composed of another materials, such as GSO
(Gd.sub.2SiO.sub.5), may be used in this invention. According to
this modification, a method of manufacturing a radiation detector
may be provide that allows provision of a radiation detector of low
price.
(5) The fluorescence detector in each of the foregoing embodiments
is formed of the photomultiplier tube. This invention is not
limited to this embodiment. A photodiode, an avalanche photodiode,
a semiconductor detector, etc., may be used instead of the
photomultiplier tube.
(6) In the foregoing embodiment, the bed is freely slidable. This
invention is not limited to this. For instance, the bed may be
fixed, whereas the gantry 11 may slide.
INDUSTRIAL UTILITY
As described above, this invention is suitable for radiation
tomography apparatus for medical uses.
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